Observations based on elution of IgG from nephritic kidneys of NZ mice and absorption of the eluted IgG with selected antigens indicate that their immune complex nephritis involves at least two kinds of antigen-antibody complexes. Antibodies reactive with nuclear antigens account for nearly half of the IgG eluted from the kidneys while antibodies reactive with Gross viral antigens make up a significant but lesser amount. Superimposed chronic viral infections affect the nephritis of NZ mice in different ways. LCM and polyoma infections hasten and intensify the antinuclear antibody responses and glomerulonephritis of these mice while LDV infection appears to protect against both antinuclear antibody formation and development of nephritis.